Tunneling and phase-sensitive studies of the pairing symmetry in Sr₂RuO₄

Results obtained from several tunneling experiments on the pairing symmetry in Sr₂RuO₄ are reported. A normal surface layer was found to be present on the ab face of single-crystalline Sr₂RuO₄ in the c-axis tunnel junctions. Measurements in the in-plane junctions, however, revealed features suggesting a suppressed, but non-zero superconducting gap and Andreev bound states (ABSs) associated only with an unconventional superconductor. Josephson coupling between a conventional s-wave superconductor and Sr₂RuO₄ has been used to probe the pairing symmetry in the latter, through both the selection rule and the phase sensitive experiments. All results indicate that Sr₂RuO₄ is unconventional, most likely spin-triplet superconductor. Finally, some recent studies of several newly-characterized ruthenates are reviewed.